Reproduction¶

Part of 4.6 Inheritance, Variation and Evolution.

Reproduction links cell biology to inheritance. The central question is how organisms make new individuals while passing on genetic information accurately enough for continuity and differently enough for variation.

Learning Objectives¶

Sexual and Asexual Reproduction¶

Asexual reproduction — advantageous when conditions are stable: no need to find a mate, and successful genotypes are copied quickly. Examples: bacteria dividing; strawberry runners; bulb division in daffodils.

Sexual reproduction — advantageous when conditions change: the genetic variation it produces may include individuals better suited to new challenges.

Some organisms can do both. The malaria parasite (Plasmodium) reproduces asexually in humans and sexually in mosquitoes. Many fungi reproduce both sexually and asexually (via spores).

Meiosis¶

Meiosis is cell division that produces gametes (sex cells). It produces four daughter cells, each with half the number of chromosomes (haploid, 23 in humans). When two gametes fuse at fertilisation, the full chromosome number (46 in humans, arranged as 23 pairs) is restored.

• Gametes are haploid (n); body cells are diploid (2n).
• Meiosis produces genetically unique cells due to:
• Chromosomes shuffling (independent assortment).
• Sections of chromosomes swapping (crossing over).

DNA, Chromosomes and Genes¶

• Chromosomes are condensed structures in the cell nucleus, each made from one long DNA molecule.
• DNA (deoxyribonucleic acid) has a double helix structure. It is composed of two strands made from four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). Bases pair specifically: A–T and C–G.
• A gene is a section of DNA that codes for a particular protein or characteristic.
• The genome is the complete set of genetic material (all genes) in an organism.

Inheritance: Alleles, Genotype and Phenotype¶

• Allele: a different version of the same gene. Many genes have two common alleles.
• Dominant allele: expressed in the phenotype whether one or two copies are present (denoted by a capital letter, e.g. D).
• Recessive allele: only expressed when two copies are present (denoted by a lowercase letter, e.g. d).
• Codominant alleles: both alleles are expressed simultaneously (e.g. blood group AB).
• Homozygous: both alleles are the same (e.g. DD or dd).
• Heterozygous: one dominant and one recessive allele (e.g. Dd).
• Genotype: the allele combination of an organism.
• Phenotype: the observable characteristics produced by the genotype (and environment).

Genetic Diagrams (Punnett Squares)¶

Monohybrid inheritance uses a Punnett square to predict the probability of phenotypes from a single gene cross.

Example — hair colour (D = dark, dominant; d = light, recessive):

If one parent is heterozygous dark (Dd) × one parent is homozygous light (dd):

Result: 2 Dd (dark) : 2 dd (light) → 1:1 ratio; 50% chance of dark or light hair.

Codominance: Blood Groups¶

Human blood groups use three alleles: I^A, I^B and I^O. - I^A and I^B are codominant — genotype I^AI^B gives blood group AB. - I^O is recessive — genotype I^OI^O gives blood group O.

Family Pedigrees¶

A pedigree diagram shows which members of a family have an inherited condition and which are carriers. They allow you to work out if a disease allele is dominant or recessive.

Example: if an unaffected pair of parents have an affected child, the disease allele must be recessive (both parents are carriers: Ff × Ff → 25% chance of ff child).

Sex Determination¶

Humans have 23 pairs of chromosomes. The 23rd pair are the sex chromosomes: - XX = female - XY = male

All egg cells carry an X chromosome. Sperm cells carry either X or Y. Therefore the father's sperm determines the sex of the child. There is a 50% chance of each sex in any pregnancy.

Inherited Disorders¶

Some disorders are caused by faulty alleles and can be passed on through families.

Polydactyly¶

• Extra fingers or toes.
• Caused by a dominant allele, so only one copy is needed for the condition to appear.
• An affected parent has at least one copy of the dominant allele; there is a 50% or 100% chance of passing it on depending on their genotype.

Cystic Fibrosis¶

• Faulty chloride channels cause abnormally thick mucus in the lungs, gut and reproductive organs.
• Caused by a recessive allele (F = normal, f = cystic fibrosis).
• Genotype ff → cystic fibrosis. Genotype Ff → carrier (no symptoms, but can pass it on).
• Both unaffected parents can be carriers (Ff) and have an affected child (ff) — 25% probability per pregnancy.

Sickle Cell Anaemia¶

• A change in the base sequence of the haemoglobin gene produces abnormal haemoglobin. Red blood cells become sickle-shaped, blocking blood vessels. Symptoms include fatigue, pain and fever.
• Caused by a recessive allele (HbA = normal, HbS = sickle).
• Carriers (HbAHbS) have resistance to malaria — an evolutionary advantage in malaria-endemic regions, explaining why the sickle cell allele is common in African populations.

Embryonic Screening¶

Embryos can be screened for genetic disorders by: - Amniocentesis — a sample of amniotic fluid from around the foetus is tested. - Chorionic villous sampling — a sample from the placenta is tested. - During IVF — a cell can be removed from the embryo before implantation and tested.

Ethical issues include: questions about termination of affected pregnancies; cost vs benefit; risk of false results; what should be done with IVF embryos found to carry serious disorders.

Common Confusions¶

• Meiosis vs mitosis: meiosis produces 4 genetically different haploid gametes; mitosis produces 2 genetically identical diploid cells.
• Carrier vs affected: a carrier of a recessive disorder has one copy of the faulty allele (Ff) and shows no symptoms, but can pass the allele on. An affected individual has two copies (ff).
• Dominant disorder inheritance: for a dominant disorder like polydactyly, an affected parent has at least a 50% chance of passing the allele to each child. You do not need both copies for the condition to show.
• Sex chromosomes: the sex of a baby is determined by which sperm fertilises the egg (X-carrying gives girl; Y-carrying gives boy) — not by the mother's egg.

Key Terms¶

• Gamete: a sex cell with half the normal number of chromosomes (haploid).
• Meiosis: cell division that produces four genetically different haploid gametes.
• Allele: a different version of the same gene.
• Genome: the complete set of genetic material in an organism.
• Dominant: describes an allele expressed when only one copy is present.
• Recessive: describes an allele expressed only when two copies are present.
• Codominance: when both alleles are expressed simultaneously in the phenotype.
• Homozygous: having two identical alleles for a gene (e.g. DD or dd).
• Heterozygous: having one dominant and one recessive allele for a gene (e.g. Dd).
• Genotype: the combination of alleles an organism possesses.
• Phenotype: the observable characteristics of an organism; determined by genotype and environment.
• Carrier: an individual who has one copy of a recessive disease allele but shows no symptoms.
• Polydactyly: an inherited condition causing extra digits; caused by a dominant allele.
• Cystic fibrosis: an inherited condition causing thick mucus; caused by a recessive allele.
• Sickle cell anaemia: an inherited condition causing sickle-shaped red blood cells; caused by a recessive allele.
• Punnett square: a grid diagram used to predict the probability of different genotypes and phenotypes in offspring.

Connected Pages¶

• 4.6.2 Variation and Evolution
• GCSE Biology Key Terms Glossary